G9 Chapter 3 Neural Processing and Perception
Play VL 3.7 – Receptor Wiring and Sensitivity as a review of end of last lecture
Other cells in the retina. Not covered specifically in G9
We’ll first follow the neural activity upwards toward the brain – along the receptor -> bipolar -> ganglion pathway. Recall the side view of the retina . . .
The Bipolar cells.
So called because it’s difficult to tell which is the “receiving” end and which is
the “sending” end. They transmit information from the receptors to the ganglion cells.
Why do they exist?
Perhaps to provide extra synapses between the receptors and ganglion cells.?? Many of these synapses are of the “comparative” type discussed in the previous lecture. That type is the basis for neural “thinking”.
The first synapse –the receptor –> bipolar synapse heightens contrast. The process by which this is accomplished is called lateral inhibition. The lateral message is carried by the horizontal cells.
The second synapse –the bipolar –> ganglion synapse may allow a completely different type of comparison, perhaps more global, perhaps wavelength comparisons.
This is pretty much the way things are in the brain – the comparisons, decisions, “thinking”, of the brain is done in the synapses.
Contrast heightening through Lateral Inhibition
Horizontal Cells – schematic view of the essential circuitry
These cells receive input from receptors and send inhibition to neighboring bipolar cells.
Since the inhibition is sent laterally, it’s called lateral inhibition.
One likely function of the horizontal cells is to heighten the contrast of borders between light and dark. Example from Figure 3.9
Each vertical gray square seems slightly darker on the left and slightly lighter on the right.
This is the distribution of light intensity
This is the distribution of brightness.
This is an example of one of the many lies our visual system tells us..
Goldstein’s description of lateral inhibition – G9 p 57
Output of each bipolar cell is the sum of the input and the total lateral inhibition.
Bipolar X: 100 – 10 – 10 = 80
Bipolar A: 100 – 10 – 10 = 80
Bipolar B: 100 – 10 – 2 = 88
Bipolar C: 20 – 10 – 2 = 8
Bipolar D: 20 - 2 - 2 = 16
Bipolar Y: 20 - 2 - 2 = 16
Lateral Inhibition may also explain simultaneous contrast G9 p 58
(To demonstrate, click on one of the squares and hit the Backspace key. Then press CTRL+Z to undo.)
G9 also mentions the Hermann Grid. I will not test you over it.
The amacrine cells
These cells receive input from bipolar cells and send inhibition (probably) to neighboring ganglion cells.
One conjecture is that the Amacrine cells are involved in inhibition associated with wavelength. That is, they’re probably involved in perception of color. It’s possible that these cells are responsible for the creation of what are called opponent processes. More on that when we study color perceptoin.
Amacrine cells may be involved in switching between rod and cone systems driving the same ganglion cell.
Ganglion cells
Only cells in retina that are clearly neurons. About 1 million per eye.
Axons form the optic nerve. So EVERYTHING WE SEE is carried to the rest of the brain over the 1 million axons of the ganglion cells in that eye. Vision is not light anymore, it’s neural activity.
This is a 100:1 compression of information – 100+ million receptors down to 1 million RGCs.
So each ganglion cell represents a composite and comparison of information from multiple receptors, bipolar, horizontal and amacrine cells connected to it.
There are over 20 specific types of retinal ganglion cells
Two most well-studied
Magni or M, larger in size with large receptive fields.
The magni ganglion cells apparently carry information about movement, location, and about gross features of the visual environment.
Parvi or P, smaller with smaller receptive fields.
The parvi ganglion cells apparently carry information about wavelength and about fine details of the visual environment.
The blind spot
The axons of the ganglion cells cross the surface of the retina and gather together at one place. There, they go toward the back of the head, out of the eye. That spot, obviously, has no receptors. It is called the blind spot.
Play VL 3.6 Visual Pathway within the Eyeball here.
Ganglion cell Receptive Fields
The collection of receptors connected to a given ganglion cell. G9 p 60
Play VL 2.9 here – it lasts about 10 minutes.
Each retinal ganglion cell isconnected to a field of receptors 100s or 1000s of them.
Those receptors make up the ganglion cell’s receptive field.
Center-Surround Characteristics of Receptive Fields G9 p 60
Most receptive fields are characterized as center surround.
Stimulation of the center yields one type of response (either an increase in activity or a decrease). Stimulation of the surround yields the opposite type of response.
This type of receptive field is called an antagonistic center surround field.
Effects of various types of stimuli on aOn-center/Off-surround receptive field. Spot of light:
Spot of light in center – positive responseSpot of light in surround – diminished response
Spot of darkness in center – diminished responseSpot of darkness in surround – positive response
Diffuse light – no response Diffuse darkness – no response for same reason
Because center and surround cancel each other
Edge of light – a response almost anywherethe edge appears, in any orientation.
Why are receptive fields organized as center-surround? This suggests that an antagonistic circular surround receptive field is an ideal shape for detecting light-dark borders of any orientation – a wonderful building block for perception.
Responding to specific stimuli G9 p 62
The circuit below consists of 7 receptors and 3 ganglion cells, connected as illustrated.
Small stimulation of the center only: .
B responds a little.
Stimulating all of the center:
B responds at a high rate
Stimulating Center and Surround:
B’s response rate is lower.
Stimulating Center and surround equally:
B responds at its base rate.
The bottom line is that Neuron B is a “3-receptor-length” detector.
It responds best when a line that is exactly 3 receptors long is presented. If the line is shorter than 3 or longer than 3, its response diminishes.
So by monitoring the output of neuron B, the visual system knows whether the bar of light striking the receptors is 3 units long or not. If B’s response is 3 or close to it, then that indicates that the bar is 3 units long. But if B’s response is lower than 3, then that indicates that the bar is not 3.
Now multiple this circuit by several million – each one responding to a different length.
Then add even more millions monitoring the output of those neurons.
Ultimately, you have the central nervous system.
G9 Chapter 3a - 1